This invention relates to methods for selectively plugging water-rich strata of subterranean formations. More particularly, the invention relates to a process of demulsifying in-situ an emulsified resin to accomplish selective plugging of water-rich strata.
In the drilling and production of fluid mineral deposits such as petroleum oil and gas from subterranean formations, presence of water has been a continuing problem. In almost every subterranean formation wherein either petroleum oil or gas (hereinafter "oil") is present in quantities which make production practical, water is also present (usually aqueous brine solutions). Usually, certain portions of strata of an oil-bearing formation may be considered oil-rich, and other portions of strata of the formation will be considered water-rich.
Production of connate water along with oil from the producing formation is not desired for many reasons, among them being the extra production expenses encountered, the necessity for separation of the oil and water following the production, and the fact that means must be provided for getting rid of undesired brackish water which is a pollutant to the surrounding terrain.
In addition, these disadvantages are magnified when certain post-primary recovery processes such as waterflood operations are utilized in such formations. In accordance with these processes, it is customary to inject a driving fluid through one or more injection wells, and to produce oil through one or more producing wells. The well fluids are pushed by the driving fluids through the formation from the area of the injection wells to the area of the producing wells. By the very nature of such processes, a relatively large amount of injected fluids will be produced at the producing well or wells. It is thus imperative that enough oil be produced through the producing wells along with the water (connate and injected) to make the process economical.
Another factor existing in many subterranean formations that seriously complicates the undesired presence of water is the existence, within the formation, of gross differences in permeability. Oil-bearing formations cannot be considered as having a homogeneous nature. It is well-known that permeability profiles of pay sand show irregularities both horizontally and vertically. For example, oil-bearing strata may be separated by shale streaks which prevent vertical migration between them and provide independent paths between injection wells and producing wells. These independent paths will likely have different effective permeabilities and generally speaking water-rich portions of strata will be the more porous strata.
These and other factors result in the watering-out of certain portions of an oil-bearing formation prior to the watering-out of other portions during water flooding. When this occurs, water passing through a watered-out strata results in oil production at increasingly unfavorable water-to-oil ratios. This water by-passing often becomes a controlling factor in determining the final recovery which may be obtained economically by waterflooding operations. This is particularly true when the oil sand is highly heterogeneous.
The shape of the reservoir which is swept by an advancing front of injected fluid may be such that large quantities of theoretically recoverable oil will not be affected by fluid flood. Thus, such oil might not be recovered because the flooding medium is preferentially channeled into other parts of the formation. Studies of reservoirs after waterflooding have indicated that two-thirds of the residual oil is frequently located in portions of the reservoir which waterflooding has not reached, while in the water-swept portions oil content may be near the irreducible minimum.
The economic significance of recovery operations of this type is great and the problem as described above has been well-recognized by the oil and gas industry for many years. Many efforts have been made to solve the problem in whole or in part.
One approach suggested in the art is to selectively plug the more porous water-rich strata so that water from such strata will not be selective produced or selectively swept by the driving fluid used in the post-primary process, or at least so that the oil-to-water ratio will be improved. The present invention is directed to this method, which is often called "selective plugging".
Various solutions have been proposed by the prior art for selectively plugging porous water-rich strata. Discussions of some of these prior art methods are found in Applicant's earlier patents, U.S. Pat. Nos. 3,749,147; 3,811,508; 3,865,189; 3,866,684; 3,866,685; and 3,876,438.
Although some of the prior art solutions have been successful under certain conditions, the need remains for other practical and inexpensive solutions to these long-felt problems. Especially the need remains for a selective plugging composition and method which will gradually plug and is effective over a large area or distance. Many of the selective plugging methods and compositions of the prior art have suffered from the defect that even though they may have plugged the water-rich portions of the well, the plugging effect was evidenced over a small area. When that happened, the driving fluid in a post-primary process was allowed to by-pass the plugged portion and then re-enter the porous water-rich strata, thus presenting the same problems as outlined above.
Because of the expense of most potential injection fluids when compared to the oil which may be recovered, many such fluids are not practical or economical for use even though they may technically be efficient. Consequently, the need especially exists for a selective plugging composition and method of the type described above which additionally is sufficiently inexpensive to justify its use.
A major criteria, therefore, in formulating a selective plugging composition is that either a small quantity of material will be effective to accomplish the plugging, or that the material utilized be very inexpensive.
The present invention satisfies the above-mentioned problems by utilization of the Jamin Effect. In 1860, the French physicist, J. Jamin, experimenting with capillary tubes containing drops of liquid interspersed with gas, found that very large pressures across the tubes were frequently necessary to cause any fluid motion. It was subsequently found that the two fluids in the tube could both be liquids providing only that the interfacial tension between them was high. The phenomenon is observable if there is only a single droplet of liquid B in the capillary otherwise filled with liquid A providing that the droplet is as large in diameter as the capillary. Given high interfacial tension, the interfacial area between A and B will be the minimum possible. Any attempt to dislodge the droplet will cause a distortion which will necessarily cause an increase in interfacial area. To achieve this increase requires that work be done on the system. This work requirement is what gives rise to the increased pressure demand.
It has been calculated (J. J. Taber, New Mexico Petroleum Recovery Research Center), that a separated droplet of oil in a reservoir can require a pressure gradient of 2 psi/foot for every dyne/centimeter of interfacial tension between the oil and the formation water in order for fluid motion to be initiated.